The present invention relates generally to an apparatus and method for the nondestructive readout of a latent electrostatic image formed on an insulating material. More particularly, the present invention is concerned with a method and apparatus for reading out the location and magnitude of accumulated charges (surface density of electrostatic charges) on a sheet or layer of insulating material which involves producing a surface depletion layer related to the accumulated charges on a sheet or layer of semiconducting material by induction and then determining the location and magnitude of the accumulated charges on the semiconductor material using the surface photovoltage effect.
The invention is especially useful in reading out a latent electrostatic image formed on an insulator by irradiation with X-rays but is not exclusively limited to electrostatic images formed by that type of radiation.
In a number of situations, such as in tire manufacturing, weaving, printing, handling liquid fuels or electronic devices, accumulated electrostatic charges are unwanted and undesirable. In other instances, such as for example in electrophotography, accumulation of electrostatic charges (i.e. static electricity), is beneficial and is used to form a latent electrostatic image of an object which is then developed. In both cases, however, it is often necessary to be able to accurately determine the location and magnitude of the electrostatic charges.
There are a variety of known methods for measuring electrostatic charge. Early techniques for sensing static electricity made use, for instance, of the gold leaf electroscope, the pith ball, and very light material such as cigarette ash. These methods have only historical value. Presently, electrostatic charge is usually determined from measurements of the electrostatic potential on a surface. This can be done, for instance, by using an electrometer (high input impedance voltmeter) to measure the ac signal induced on a reference electrode. The ac signal may be generated in this method by periodically introducing an electrical shield into the space between the reference electrode and the surface under measurement. The electrometric method is nondestructive and allows for measurement of a magnitude of electrostatic charge. However, the determination of the distribution of charge requires slow and cumbersome mechanical scanning of the studied surface with a small aperture reference electrode or the use of an array of electrodes.
There exist a number of destructive methods for determining charge distribution. Classical examples are the vidicon tube and electrophotography. In a vidicon tube, a charge distribution pattern is stored in a semiconductor target. The distribution of charge is determined by measuring the variation of electron beam current when the target is scanned by an electron beam. In electrophotography, the charge distribution on a xerographic plate (i.e. the latent electrostatic image formed on the xerographic plate) is determined from the distribution of toner particles which are attracted to the xerographic plate by the charges during the development process.
The prior art of nondestructive measurement of charge accumulated in an insulating layer using semiconductors lies largely in the area of electronic devices, especially computer memories. In this case determination of charge stored in a single element is accomplished by measuring variations of current in the conductive channel formed under the surface of the semiconductor.
The subject of ac surface photovoltage is described in a paper by E. O. Johnson of RCA Laboratories, entitled, "Large-Signal Surface Photovoltage Studies with Germanium", Physical Review, Vol. 111, No. 1, pp. 153-166, 1958. The paper discusses the relation between surface photovoltage and surface potential and hence space charge in the semiconductor.
The photovoltaic response of the semiconductor InSb has been used for determination of the charge distribution induced in the semiconductor by an electromagnetic radiation. This is described by R. J. Phelen, Jr., and J. O. Dimmock, in an article entitled, "Imaging and Storage with a Uniform MOS Structure", Applied Physics Letters, Vo. 22, No. 11, pp. 359-361, 1967. The image projected on a uniform MOS structure (a semitransparent metal film-oxide layer-InSb sandwich) modified the surface depletion region in the semiconductor. The charge stored in the depletion layer was determined by measuring the photovoltaic response resulting from saturation of this layer by a "reading" photon beam. The few micron thick depletion layer is the only active structure.
More recently, it has been shown that the ac surface photovoltage induced by a low intensity beam of light, modulated at high frequency, and having photon energy comparable to or exceeding the band gap of the semiconductor, is proportional to the reciprocal of the semiconductor depletion layer capacitance and hence is proportional to the density of charge in this layer. Furthermore, it has been found that under proper conditions the measured signal is only weakly dependent on the distance between semiconductor and the reference electrode. This is discussed by E. Kamieniecki in a paper entitled, "Determination of Surface Space Charge Capacitance Using Light Probe", Journal of Vacuum Science & Technology, Vol. 20, No. 3, pp. 811-814, 1982; and a paper entitled "Surface Measured Capacitance: Application to Semiconductor/Electrolyte System", Journal of Applied Physics, Vol. 54, No. 11, pp. 6481-6487, 1983.
The general conclusion of studies made to date is the existance of a correlation between the local magnitude of charge in the depletion layer at the semiconductor surface and the ac surface photovoltage. The ac surface photovoltage is defined herein as the variations of the surface potential induced by a photon beam which is intensity modulated, either periodically or not periodically. This photon beam may cause the generation of carriers at the front surface in the depletion region, or, when illuminated from the back (opposite side) in the bulk and diffusion (migration) of the carriers toward the depletion region.
In U.S. Pat. No. 3,859,527 to G. W. Luckey there is disclosed an apparatus and method for recording images on recording mediums which images correspond to high energy radiation patterns. A temporary storage medium, such as an infrared-stimulable phosphor or thermoluminescent material, is exposed to an incident pattern of high energy radiation. A time interval after exposure a small area beam of long wavelength radiation or heat scans the screen to release the stored energy as light. An appropriate sensor receives the light emitted by the screen and produces electrical energy in accordance with the light received. The information carried by the electrical energy is transformed into a recorded image by scanning an information storage medium with a light beam which is intensity modulated in accordance with the electrical energy.
Articles of interest concerning gas ionography, sometimes referred to as electron radiology, wherein X-ray photons are absorbed in a high-pressure gas between the parallel plates of an ion chamber and the ions produced are collected on an insulating foil covering one of the plates include "Efficiency and Resolution of Ionography in Diagnostic Radiology" by A. Fenster, D. Plewes and H. E. Johns in Medical Physics, Vol. 1, No. 1, 1974, pages 1-10; "Gas Ionization Methods of Electrostatic Image Formation in Radiography" by H. E. Hohns etc., British Journal of Radiology, September, 1974, pages 519-529; "Charging Characteristics of Ionographic Latent Images", B. G. Fallone and E. B. Podgorsak, Medical Physics, 11(2), March/April, 1984, pages 137-144; "Liquid Ionography For Diagnostic Radiology", A. Fenster and H. E. Johns, Medical Physics, Vol. 5, No. 5, September/October, 1974, pages 262-265; and Theoretical and Experimental Determination of Sensitivity and Edge Enhancement in Xeroradiography and Ionography" D. Plewes and H. E. Johns, Medical Physics 7( 4), July/August, 1980, pages 315-323.